Astronomy

According to the CSET website, there will be 7 multiple choice questions and no Constructed Response Question.

a. Describe the chemical composition and physical structure of the universe

Chemical Structure-The gas/chemicals that give birth to the solar systems are the product of billions of years of galactic recycling. The whole universe is thought to have been born in the Big Bang. The Big Bang produced two chemicals, hydrogen and helium. Heavier elements were produced later by stars through nuclear fusion or the explosion of the star during its death. These elements can be recycled into new generation of stars. Heavier element content increase with each generation of massive stars. The chemical composition of the galaxy has remained predominately hydrogen and helium. Chemical composition of the universe is roughly 74% hydrogen and 24% helium by mass.Other elements are in trace amounts.

Physical structure- Just within the observable universe, scientists estimate that there are 10,000,000,000,000,000,000,000 or 10^22 stars! The universe is still expanding, forming new stars, and contains decaying star corpses. The universe is very large, perhaps infinite in volume. The observable matter is spread out over a space of at lease 93 billion light years across. There are probably more than 100 billion galaxies in the observable universe. The observable matter in the universe may be condensed into stars, which are part of galaxies, which may be part of a cluster (groups of galaxies with a few dozen members). Each region of the sky contains roughly the same content. The most precise estimate of the universe's age is around 13.7 billion years old at the current microwave temperature of 2.73 K.

b. Describe the structure of the solar system and its place in the Milky Way galaxy

The solar system formed from a cloud of gas, called the solar nebula, that collapsed under its own gravity. The solar nebular is thought to have begun as a large and roughly spherical cloud of cold gas. The collapse of the nebular is thought to perhaps start due to a nearby event such as shock waves from a nearby star exploding. The nebular began to collapse and gravity began to pull in all direction, which explains why the sun and planets are spherical. As the nebula shrank, the temperatures began to rise (energy conservation). The sun formed in the center of this nebula where temperature and density was the greatest. As the solar nebula shrank, it began to rotate faster and faster (conservation of angular momentum). The rapid rotation helped to ensure that not all of the materials collapsed into the center of the solar nebula. As the particles collided in the spinning cloud, the nebula began to flatten into a disk. The spinning disk helps to explain why the planets all orbit the sun in the same plane and direction.

Other features:1) Large bodies in the solar system have orderly motions- all planets have nearly circular obits moving in the same direction (the same direction as the sun's rotation).

3) asteroids/comets: vast number or rocky asteroids and icy comets are found throughout the soar system, but are concentrated in three regions (asteroid belt, Kuiper belt, and Oort cloud.

4) Exception: some planets have unusual axis tilt, unusually large moons, or moons with unusual orbit, or unusual rotation (such as Venus).

5) Because we live in our Milky Way galaxy, it is hard to know exactly how it looks and its structure. We do know that our galaxy holds 100 billion stars. It is a spiral galaxy. From the side, it is flat with a bulge in the center. The entire galaxy is about 100,000 light years in diameter. Our solar system is located in the disk about 28,000 light years from the milky way center.

Astronomical Unit: the average distance of the Earth from the sun, which is about 150 million Km.

Cosmic Address: a way to think of Earth's place in the universe. Earth > Solar System > Milky Way Galaxy > Local Group > Local Supercluster > Universe

c. Distinguish between stars and planets

Stars- Stars are born when gravity is able to contract the cloud of cold interstellar gas to the point where it becomes so hot that nuclear fusion begins in its core. The colder this cloud, the easier it is for gravity to contract the gas as lowering the temperature of this cloud reduces its gas pressure. Cold, dense clouds that form stars are known as molecular clouds (called so because they allow hydrogen atoms to combine to form hydrogen molecules). The molecular clouds also tend to be massive and generally can give birth to many stars at a time (stars are generally born in clusters).Chemical composition of a star's mass at birth is about ¾ hydrogen and about ¼ is helium. Stars generates heat and light through nuclear fusion in its core. Even though stars are not living organism, they go through life cycles. Stars are born when gravity compresses the material in a cloud to the point where the center becomes hot and dense enough for nuclear fusion to take place. Blue stars are hotter and red stars are cooler (but bright). Spectral sequence stars that fuse into helium in their cores all fall on the main sequence category. Stars that do not fuse hydrogen into helium are off of this sequence. The corona is the outermost layer of the star's atmosphere and has an astonishingly high temperature, around 1 million K. This high temperature explains why this region emits most of the Sun's x-rays. This layers density is so low, however, that if you were to fly your spaceship through this layer, it would feel relatively little heat. The chromosphere is the middle layer of the solar atmosphere that radiates most of the Sun's UV radiation. This layers temperature is around 10,000 K.The photosphere is the lowest layer of the Sun's atmosphere and is the visible surface of the Sun. Sunspots are found on this layer. Convection zone is the central region of the Sun where energy is transported outward by convection. Radiation zone Energy energy moves outward as photons of light. THe temperatre rises to around 10 million K. Core is where nuclear fusion is taking place, transforming hydrogen into heliu. The temperutre is around 15 million K and ists density is more than 100 times that of water. The pressure is 200 billion times that on the Earth's surface. It will take a few hundred thousand years for the energy that is produced in the core to reach its surface.

Quick Quiz!1) If a star 300 light years away were to explode (supernovae) today, we would:a) be able to observe it with our eyes that nightb) affect our satellite and electronic equipments and would only be able to observe it through strong telescopesc) would not be able to observe it for another 300 years until the light from this event reaches Earthd) would only be able to observe it using x-ray telescopes

Correct Answer: C If a star that was 300 light years away were to explode (supernovae), it would take 300 years for that star's light to reach Earth.

Correct Answer: C Stars are born in cold, dense clouds of gas whose pressure cannot resist gravitational contraction. When gravity causes a cloud of interstellar gas to contract and it becomes hot enough, then nuclear fusion will begin to take place in its core. In option A, dark matter is matter that we infer to exist from its gravitational effects, but haven't been able to detect any light. In option C, cumulus is a type of cloud found in Earth's atmosphere. In option D, superclusters consists of many clusters or groups of galaxies. They are the largest known structures in the universe.

Planet- planets are moderately large objects that orbits around a star. Planets are large enough to have gravity to make it round. Planets orbit within the same plane and in the same direction. In addition, to be a planet, the orbit cannot cross any other planets path. When our solar system was “born”, gravity in the center drew in material that formed the sun. The gaseous material was too spread out and began to clump together until it reached a certain size, where gravity could then start pulling these clumps together to form a planet. As temperature began to drop, gaseous rock and metal materials could condense into a solid. Hydrogen compounds could condense into ice beyond the frost line (between Mars and Jupiter). Within the frost line, only terrestrial planets could form (only rock and metal rock could condense into solid “seeds”. Beyond the frost line, hydrogen compounds condensed into ice which allowed ice along with metal and rock to build upon.

In 2006, the IAU (International Astronomical Union) decided that in order for a celestial body to be considered a planet, it must meet three requirements:1. It orbits around the Sun2. It has enough mass to generate a gravity strong enough to make it round (hydrostatic equilibrium)3. Must clear its neighborhood as it orbits the sunAs a result of these rules, because Pluto only met two of these requirements, number 1 and 2, it was reclassified as a dwarf-planet.

Additional information about the Earth: If you were to place Earth on an imaginary plane, you would see that it is tilted at an angle of 23.5 degrees. The axis is currently pointing towards the star, Polaris (aka, the North Star). As the Earth revolves around the sun, there is a slight wobbling action that takes place. Think of one of those top toys that you spin. As it rotates, its tip swings from one region to the other side. The Earth goes through this as well, but at a much slower rate. This is known as precsesion and is the result of the gravitational tugs from the sun and the moon. Each cycle of Earth's precession takes about 26,000 years and slowly changes where the axis points in space.

d. Recognize that stars vary in color, size, and luminosity

Color- stars come in almost every color of the rainbow. Colors of the stars tells us something about their surface temperature. Stars come in different colors because they emit thermal radiation. The thermal radiation spectrum depends on the surface temperature of the star. Astronomers classify stars according to their surface temperature by assigning a spectral type. The order of the spectral types goes from hottest to coolest, OBAFGKM (Oh Be A Fine Girl Kiss Me). The H-R diagram became a very important tool in displaying star's surface temperature (spectral type) and its luminosity. A general trend when observing stars' luminosity and color is that the brightest and hottest stars are white with some blue. The coolest dim stars are reddish. Stars, such as our Sun, are more modest in temperature and are yellowish-white.

Size- Weighing a star is much more difficult than measuring its surface temperature or luminosity. Astronomers use Newton's version of Kepler's third law in order to measure a stars weight.

Kepler's third law: "The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit". In other words, distant planets orbit the sun at slower average speeds.Kepler did not know how this law worked, he just knew it did. Another issue with this law is that it doesn't work for objects that do not orbit the sun, such as the Earth's Moon. Newton was able to solve both issues with his Theory of Gravity by identifying that the masses of orbiting bodies play a part. Newton adapted Kepler's Third Law so that it can apply to any two orbiting objects with a common center of mass. Thus, in order to use Newton's version to measure the mass of a star, we need to use two stars that are in a binary star system. We also need to know the orbital period and the separation of the two stars. To measure an orbital period: 1) in a visual binary star system (one that we can see with a telescope) can be measured by observing how long each orbit takes. 2) in an eclipsing binary star system (star system that orbit in our line of sight. when one star eclipses the other, their apparent brightness drops. When neither star is eclipsed, then we see the combined light from both stars) can be measured by measuring the time between eclipses. 3) in a spectroscopic binary system (neither visual or binary, use Doppler shifts in its spectral lines), we measure the time it take for the spectral lines to shift back and forthMeasuring separation is more difficult. Rarely, astronomers are able to measure the separation directly. In other cases we measure it using their Doppler shifts. When studying an eclipsing binary star system (they lie in our line of sight), we can use their Doppler shift to tell us their orbital velocities. Using eclipsing and other types of binary star systems, astronomers have been able to identify the mass of may different stars. The size must be between 150Msun (150 times the mass of our Sun) and 0.08Msun (0.08 times the mass of our Sun). If the star were larger than 150Msun than the energy output from the core would drive the outer layers out into space. On the other hand, if the star were to be smaller than 0.08Msun, than its gravity would be too weak to contract the core to where it could reach 10 million K needed to fusion to take place. If this occurred, it is known as a “failed star” and is called a brown dwarf. Low mass stars of small-medium size tend to live long lives. Stars that are bigger than the sun have main-sequence lifetimes shorter than the suns 10 billion years and stars smaller than the sun have a longer life. The larger the star, the faster the rate of nuclear fusion takes place, thus the sooner hydrogen is being used up.Giants and supergiants are the red stars that are found to the right of the main-sequence in the H-R diagram. The color red indicates that they are cooler, but these types of stars are much more luminous than our Sun. In order to be very bright, they must have a very large surface area. These giants are near the end of their lives as they have already exhausted hydrogen as their fuel for fusion. As a result, these stars try to avoid gravity from crushing it together and so it begins a furious fusion that causes the star to expand to an enormous size.

Luminosity- Luminosity is the total amount of power energy per second that a star emits into space regardless of their distance. Luminosity is different from apparent brightness as apparent brights is the amount of light from a star that reaches earth (energy per second per square meter). Apparent brightness is how bright a star appears to our eyes. Think about a 70-watt light bulb. Its luminosity is always constant, however its apparent brightness will change depending on your distance from this light bulb. The closer the bulb is, the brighter it will appear. When measuring a stars luminosity, we need to know its distance from Earth and its apparent brightness. Astronomers assign each star a luminosity class, using Roman numerals I to V. Luminosity class is actually more closely related to is size than its luminosity and tells us about its radius. Class: Descriptions:I These stars have the largest radii and are known as supergiants. As we move down the classes, the star's radii decreases. II Bright giants. III GiantsIV SubgiantsV Main-sequence stars.

Thus, when astronomers classifies a star, they use the spectral type and luminosity. For example, our Sun's spectral type is G2 and its luminosity class is V. Thus, a G2 V star is a star that is yellow-white, and is a main-sequence star that is fusing hydrogen. Betelgeuse is an M2 I star. This means that it is a red supergiant star that is dim due to its small size and is no longer fusing hydrogen.

Distance- Distance can be measured through a direct method called the stellar parallax. This is the small annual shift in the star's apparent position cause by Earth's motion around the Sun. You can actually observe parallax by holding your finger at arm's length and then looking at it alternately with first one eye close and then the other. Astronomers measure stellar parallax by comparing observations of a nearby star made 6 months apart. This nearby star appears to shift against the background of the more distant stars because we are observing it from two opposite points of the Earth's orbit. Astronomers are able to calculate a star's distance if they knew the precise amount of the star's annual shift due to parallax. The parallax angle needs to be measured in order to calculate a star's distance. This angle would be smaller if the star were farther away, so distant stars have smaller parallax angles.

All stars are actually so far away, that they have very small parallax angle. The nearest stars have parallax angles smaller than 1 arcsecond. Current technology allows astronomers to measure parallax accurately only for stars within a few hundred light-years. A parsec is the distance to an object with a parallax angle of 1 arcsecond. This is equivalent to 3.26 light-years. The word parsec comes from combining the words parallax and arcsecond. To find a star's distance is to measure the parallax angle p in parcseconds, the starts distance, d, in parseccs is d=1/p. Then you multiply by 3.26 to convert from parsecs into light-years. So, an example is if a star had a parallax angle p=1/10 arcsecond, it would be 10 parsecs away, or 10 x 3.26 = 32.6 light-years.

e. Describe a simple model of how fusion in stars produces heavier elements and results in the production of energy, including light

Fusion that takes place in a star involves combining, or fusing, two or more small nuclei into a larger elements. Within the sun, temperatures reaches to around 15 million K and is under extreme pressure. The nuclei move at such a high speed pace, due to the high temperature. Electromagnetic forces repels the nuclei, preventing collisions. When two nuclei collide, it produces a heavier nucleus. The only force that overcomes this electromagnetic repulsive force that binds protons and neutrons is the strong force. The high temperature within the star allows the nuclei to travel at very high speeds, allowing them to come close enough to fuse. The higher the temperature, the harder they will collide, increasing the chance for fusion. The pressure keeps the hot plasma together, keeping the star from exploding into space. If heavier elements need to be created, higher temperatures are needed. It takes approximately hundreds of thousands of years for the photon energy to travel from the core to the photosphere!When four hydrogen nuclei combine to produce a helium nucleus, a little bit of mass is converted into energy. The sequence of steps that occurs in the sun is called the proton-proton chain. There are three steps:Step 1: Two protons fuse together to form a deuterium nucleus. A deuterium nucleus consists of 1 proton and 1 neutron. This step takes place twice to form two deuterium nuclei.Step 2: The deuterium nucleus and a proton fuse together to form the nucleus of helium-3. Helium-3 consists of 2 protons and 1 neutron. This step also takes place twice.Step 3: Two helium-3 nuclei fuses together to form helium-4. Helium-4 consists of 2 protons and 2 neutrons. Two excess protons are released in this process.

The solar energy that is formed as a result of fusion takes hundred of thousands of years to travel from the core of the sun to the photosphere. Even though photons travel at the speed of light, their route through the sun takes on a zigzag path, so much so that it takes them a very long time to make any progress. This is because the plasma in the sun is so dense, that photons can only travel a fraction of millimeter in any one direction before it "collides" with an electron, which causes it to deflect into another random direction.

f. Describe the regular and predictable patterns of stars and planets in time and location

If you are able to get away one evening from the city lights, you will be able to view more than 2,000 stars as well as the whitish band of light, the Milky Way. Observing the sky night after night will allow you to recognize some patters in the stars. These patterns have not changed noticeably in the past few thousand years. Patterns formed by stars that we see in the night sky are called constellations. Bright stars help us to identify constellations. 88 official constellations were chosen in 1928 and which ones you see in the sky depends on the latitude and the time of the year. Constellations are an illusion. The stars may appear to lie close to each other, but in actuality may be quite far apart. This illusion occurs because we lack depth perception when we look up into space. The ancient Greeks viewed the constellations as we do and believed that they were indeed lying on a great celestial sphere that surrounded the Earth. While we do understand in this modern age that starts do not lie in a celestial sphere, we can use this to help us understand and map the sky. We use four special points: 1) north celestial pole that points directly over Earth's North Pole2) south celestial pole that points directly over Earth's South Pole3) Celestial equator is the projection of Earth's equator into space, making a complete circle around the celestial sphere4) ecliptic is the path that the Sun follows as it circles around the celestial sphere, crosses the celestial equator at a 23 1/2 degree angle

Standing outside, you will also notice that it feels like the universe is circling around us. The stars moving gradually across the sky from east to west. Going back to the celestial sphere, it appears that every object in the celestial sphere is making a daily circle around Earth. The stars near the north celestial pole remain above the horizon, never rising or setting but instead making daily counter-clockwise circles around the north celestial pole. These stars are circumpolar. Stars near the south celestial pole never rise above the horizon at all. The other stars have these daily circles that are partly above the horizon and partly below it. As Earth is rotating from west to east, this stars then appear to rise in the east and set in the west.

g. Explain and predict changes in the moon’s appearance (phases)

The moon's phases are based on its position relative to the sun as it orbits the Earth. As you look at the moon as it orbits Earth, you see different combinations of light and dark regions on the moon. Each complete cycle starting with the new moon takes 29 ½ days. In addition to the moon's appearance changing as it orbits Earth, its rise and set times also change.

For example, during its full moon phase, when it is opposite to the sun in the sky, it rises around sunset, reaches it high point at midnight, and sets around sunrise.

New moon- The moon lies close to the sun in the sky and so it is hidden by the sun's bright light. It rises at 6 in the AM, highest at noon, and sets at 6 PM.

Waning Crescent- We observe a sliver of the moon starting to “shrink”. It rises at 3 AM, highest at 9 AM, and sets at 3 PM

Third Quarter- Half of the moon is lit now. It rises at midnight, highest at 6 AM, and sets at noon.

Waning Gibbous- The moon continues to “shrink”. It rises at 9 PM, highest at 3 AM, and sets at 9 AM.

Full Moon- The moon is opposite from the sun. Rises at 6 PM, highest at midnight, sets at 6 PM.

Waxing Gibbous- moon is starting to “grow”. It rises at 3 PM, highest at 9 PM, and sets at 3 AM.

First Quarter- the moon rises at noon, highest at 6 pm and sets at midnight.

Waxing Crescent- rises at 9 am, highest at 3 pm, and sets at 9 pm.

The order of the moon's phases are as follows: New Moon > Waxing Crescent > First Quarter > Waxing Gibbous > Full Moon > Waning Gibbous > Third Quarter > Waning Crescent > Back to New Moon

Quick Quiz!Based on the position of the Sun, Earth, and Moon in this illustration, which phase of the moon would you witness on this night?

a) full moonb) half moonc) gibbous moond) new moon

Correct Answer: A As the moon orbits the Earth, it goes through several different phases. The full moon occurs when it is far from the Sun in the sky. It is a bright full moon (sometimes associated with the strange behaviors that our students exhibit!). As it continues to orbit the Earth, it is said to be waning (decreasing). Waning crescent would occur when the moon is orbiting closer to the Sun in the sky. When the moon is closest to the Sun, it is in the New moon phase. We cannot see the moon during this phase as it is hidden from view by the bright light of the Sun. As the moon continues to orbit, it is going through the waxing (increasing) phase. Waxing gibbous occurs right before the Full moon phase.

Lunar Eclipse: this occurs when the Earth lies directly between the Sun and the Moon. The Earth's shadow falls on the Moon. Solar Eclipse: this occurs when the Moon lies directly between the Sun and the Earth. The Moon's shadow falls on Earth.

h. Describe the use of astronomical instruments in collecting data, and use astronomical units and light years to describe distances.

Telescopes- Telescopes allow scientists to analyze the light the telescopes collect. Using this data, scientists can help reveal the chemical composition, temperature, or even the speed of an object light years away. In order to study other aspects of space, scientists use other telescope instruments to capture images of different wavelengths. And, if you've wondered why scientists haven't invented one telescope that could capture all of the different wavelengths, it is due to the fact that the type of wavelength we are studying dictates the build of the telescope. To have one that will capture all is just not possible, unfortunately.

Radio Telescopes- studies wavelengths that are much longer than visible light waves. Very large radio telescopes are needed in order to achieve good angular resolution. The structure of these telescopes are vary in design, size, and configuration, but are typically very large, parabolic shaped antennae. They are built far from cities to avoid detection of artificial radio signals. Karl Guthe Jansky was the first to discover radio waves emitting from the Milky Way Galaxy. Discovered pulsars.

Infrared: studies objects that emit an infrared wavelengths (temperature of objects must be above absolute zero)

Visual: There are two main features of a telescope: light-collecting and its angular resolution. The light-collecting area tells us how much light a telescope is able to collect at a time. Angular resolution tells us the smallest angle that two stars are distinct. In other words, our own eyes has an angular resolution of about 1 acriminute. So, if two stars in the sky are separated by less than 1 acriminute, then our eyes do not see them as being two individual stars and we will see them as one single star. Some telescopes have an angular resolution of about .05 arcsecond. Larger telescopes have even smaller angular resolution. Telescopes come in two designs: refracting and reflecting. Refracting works much like our eye. It uses transparent glass lenses to focus on light from distant objects. Reflecting telescopes uses mirrors to collect and gather light.

X-Ray telescopes- x-ray telescopes measure X rays in space.

Gamma ray Telescopes: Used to detect the very short, high energy wavelengths of gamma rays. Sources of cosmic gamma-rays are extremely weak and require long observations in order to obtain accurate measurement of the source.